1. Field of the Invention
The present invention provides a process for the production of a superior solvent for use in a coal liquefaction process.
2. Brief Description of the Prior Art
In conventional coal liquefaction processes, particulate coal is treated at an elevated temperature and pressure in the presence of a hydrogen-donor solvent and hydrogen gas in order to remove the mineral matter, lower the sulfur content of the coal and convert the coal into liquid and low melting temperature solid products. These products also can be upgraded through catalytic hydrogenation to produce higher quality liquids.
Presently, little is known of the exact mechanisms by which the coal is transformed into soluble form or of the detailed chemical structure of the soluble coal products. It is known that many coals are easily solubilized and for others solubilization is more difficult. Some correlations have been made between the rank of the coal and ease of the solubilization and product yield. Little is known about the relationships to product quality.
The initial products produced by a coal liquefaction process may have utility as a substitute clean fuel or boiler fuel. However, for substitute fuels of higher quality, specifications on viscosity, melting point, ash, hydrogen and sulfur contents are much more stringent. Attempts to meet these specifications by operating the coal liquefaction process under more severe operating conditions have met with many difficulties. For example, the processes have experienced low liquid yields, high hydrogen consumption, difficulty of separating unreacted residue and excessive char formation which often completely plugs process transfer lines and reactors.
Alternative methods of improving product specifications through catalytic hydrogenation also are difficult. The problems which arise include: (i) a susceptibility of the liquefaction products to polymerize and deposit as coke on catalyst used for their conversion; (ii) fouling of catalyst by physical blockage as the molecular size of the coal liquefaction products approach the pore size of conventional catalysts; and (iii) catalyst contamination by metal contaminants or other nitrogenous and sulfur compounds contained in the coal liquefaction products.
In the process of converting the coal to liquid and low sulfur, low melting temperature solids, several reaction steps occur. Generally, the coal is admixed with a suitable solvent and hydrogen and the admixture is passed through a preheater to raise the reactants to a desired reaction temperature and pressure. For bituminous coal, the coal is substantially dissolved by the time it exits the preheater. Subbituminous coals also can be dissolved, but care must be exercised not to raise the temperature level above that at which excessive charring occurs. The products exiting from the preheater then are transfered to a reactor where further conversion takes place to lower the heteroatom content of the dissolved coal to a specification sulfur content and melting point temperature. The products exiting the reactor initially are flashed. The flashing depressurizes the product stream and removes gases and light organic liquids. The remaining product stream then is distilled to recover the solvent and liquid products having a boiling point temperature similar to that of the solvent. The solvent and a sufficient quantity of the separated liquid products to supplant any solvent consumed or lost in the process are recycled. The liquefaction product remainder can be further treated to separate the liquefaction products into several fractions.
The precise chemical nature of the coal liquefaction products still are unknown. Generally, their composition is discussed in terms of solubility. Several classifications commonly are used, these include: (i) oils, which are organic compounds that are hexane or pentane soluble; (ii) asphaltenes, which are benzene soluble; and (iii) cresol soluble-benzene insoluble materials which often are referred to as preasphaltenes.
It is known that the composition of the liquids and solids produced during a coal liquefaction process depend not only upon the nominal process operating conditions such as temperature, pressure and residence time, but also upon the solvent employed. It is known that a "good" solvent reduces the production of preasphaltenes and increases the production of either or both oils and asphaltenes.
Whatever coal liquefaction process is employed to convert the coal to coal liquefaction products requires the use of an extraneous solvent for process start-up. This extraneous solvent is slowly replaced in the process by a coal-derived recycle solvent. It is desirable to provide a start-up solvent that will permit the production of an optimum coal derived product for subsequent use as process solvent. However, the selection and production of such a optimum start-up solvent is very difficult to achieve. It would be desirable to provide a means by which a start-up solvent can be produced which will permit production of a desired coal-derived recycle solvent stream.